When form does not predict function: Empirical evidence violates functional form hypotheses for marine macroalgae

Functional groups are widely used to reduce complexity and generalize across ecological communities. These models assume that shared traits among species correspond to some ecological role, process or function, and that these traits can be leveraged to generate meaningful and distinct functional groups so that intergroup trait variation exceeds intragroup variation. We sought to validate the assumptions of the widely used functional group model (FGM) for marine macroalgae, which groups species based on morphological complexity, by testing the predictions of the FGM for several traits assumed to correspond with morphological complexity. The FGM predicts increased resistance to disturbance and herbivory as morphological complexity (tensile strength and thallus toughness, respectively) increases. The FGM also predicts a trade‐off between complexity and growth rate. To test predictions, we measured (a) thallus toughness (force to penetrate), (b) tensile strength (force to break) and (c) relative growth for both tropical and temperate macroalgae from different functional groups. Thallus toughness followed model predictions at the functional group level, though there was significant variability among species. However, the model did not predict tensile strength at any level for either tropical or temperate macroalgae. Furthermore, relative growth did not follow predictions; rather it was highly variable among species and functional groups. Synthesis . The assumptions of the FGM that differences in morphological complexity can be used to generate distinct functional groups and that intergroup trait variation outweighs intragroup variation were violated, providing strong evidence that individual species responses need to be considered. Furthermore, violations of assumptions indicate that functional groups should not be used to predict community responses to ecological drivers and/or species contributions to ecosystem function. Our study challenges the usefulness of functional form groups for marine macroalgae and emphasizes the need for a different conceptual framework.